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from the omnes-baryonia-est-divisa-in-partes-tres dept.
Experimental physicists Tomasz Skwarnicki, professor of physics in the College of Arts and Sciences at Syracuse University, assisted by Liming Zhang, an associate professor at Tsinghua University in Beijing, analyzed data from the Large Hadron Collider and uncovered three new pentaquarks.
(Normal matter particles, or baryons, consist of three quarks. Pentaquarks are subatomic particles consisting of four quarks and one antiquark. The anti-quark and one quark offset each other creating an exotic baryon particle comprised of five quarks.)
What is unique about each of these three pentaquarks is that its mass is slightly lower than the sum of its parts—in this case, the masses of the baryon and meson. "The pentaquark didn't decay by its usual easy, fall-apart process," Skwarnicki says. "Instead, it decayed by slowly and laboriously rearranging its quarks, forming a narrow resonance."
Skwarnicki notes
"Pentaquarks may not play a significant role in the matter we are made of," he says, "but their existence may significantly affect our models of the matter found in other parts of the universe, such as neutron stars."
There is also a theorized stable pentaquark that could conceivably be produced in appropriately energetic situations, so there might even be a few laying about.
(Score: 4, Interesting) by PiMuNu on Wednesday March 27 2019, @11:36AM
> its mass is slightly lower than the sum of its parts
I think they mean that the decay has to proceed via a weak interaction process similar to beta decay (or possibly some second order strong process, it isn't quite clear from the article). This means that the decay probability is small, so lifetime is long.
> forming a narrow resonance.
Naively, this follows from Heisenberg Uncertainty Principle. Energy and time are conjugate, so if the particle lifetime is long then the mass of the state can be well defined; so we only see the state appearing in a narrow range of energies. LHCb measures the energy of the decay products and from this reconstructs the energy of the pentaquark (or whatever). We see a spike at a particular energy indicating that collisions are more likely to occur at that energy; and by looking at the range of energies, we get a "resonance width".